1. Field of the Invention
The present invention relates to a flow-velocity measuring device for detecting the velocity of a fluid.
2. Description of the Related Art
In electronic equipment such as personal computers in which air cooling is performed by a cooling fan, lowering of the quantity of an airflow due to clogging of a filter causes lowering of a cooling capability and hence may cause failure of function thereof. Therefore, such equipment is adapted in such a manner that the quantity of the airflow is constantly observed by the flow-velocity measuring device, and when the quantity of the airflow is lowered, the number of revolutions of the fan is increased, or an alarm is given to a user. In the related art, the flow-velocity measuring device used for such an application has a structure in which a sensor element is arranged in a flow channel to measure the flow velocity of air. However, when the flow-velocity measuring device in the related art is used for a long time, there arises a problem such that dust or grim in the air is attached to and accumulated on the sensor element, and hence the accuracy may be lowered.
In order to solve the above-described problem, Patent Document 1 (Japanese Patent No. 3124457) discloses a flow-velocity measuring device in which a trap wall is provided in a flow channel, so that the airflow is introduced to a sensor element after having caught dust in the airflow by a trap wall utilizing inertia of the dust. However, there is a disadvantage such that the dust may be accumulated in the device, and in due course, clogs the flow channel, or a risk such that when the equipment provided with the flow-velocity measuring device is carried, the dust accumulated in the device may attach to the sensor element. There is also a problem such that the length between an inlet port and a discharge port is long, and hence it cannot be installed in a short flow channel.
In Patent Document 2 (U.S. Pat. No. 4,914,947), there is disclosed a flow-velocity measuring device in which an arcuate flow channel and a branch channel branched from the arcuate flow channel inwardly of the arc are defined by providing a groove in a main body, so that dust contained in a fluid is gathered to the outside of the arc of the arcuate flow channel by a centrifugal force (inertia force), and the fluid containing less dust inside the arc is introduced to the branch channel, thereby obtaining flow of the fluid with less dust containing therein, and the velocity of the fluid is detected by a sensor element. The flow-velocity measuring device in Patent Document 2 is adapted to separate the dust utilizing a gravitational force when the flow velocity is low by providing steps so that the depth of the branch channel is smaller than that of the arcuate channel. However, according to the flow-velocity measuring device in Patent Document 2, since the flow channel is defined by closing an upper opening of the groove on the main body by a circuit board having the sensor element, it is not possible to provide a step between the arcuate channel and the branch channel on the side of the circuit board. Therefore, when the device is installed upside down, there arises a problem such that the dust cannot be separated sufficiently when the flow velocity is low. In addition, since the measuring accuracy may be deteriorated if leakage exists between the circuit board and the main body, it is necessary to place the packing between the circuit board and the main body in order to realize the flow-velocity measuring device of high accuracy, which may result in a complicated structure. Furthermore, the flow-velocity measuring device in Patent Document 2 is disadvantageously long in the direction of flow of the measured fluid as in the case of the flow-velocity measuring device in Patent Document 1.
The flow-velocity measuring device is desired to be short in size in the direction of flow of the measured fluid and small in surface area which extends at a right angle with respect to the direction of flow for blocking the flow in order to downsize equipment in which the flow-velocity measuring device is integrated. Therefore, in the case of the flow-velocity measuring devices in Patent Documents 1 and 2, since the inlet port is located apart from the discharge port, even though the size in the direction of flow can be reduced, the surface area for blocking the flow increases disadvantageously simply by opening the inlet port and the discharge port laterally.